Antibody testing has accelerated in the United States in recent weeks: In one prominent study, for example, involving some 3,000 New Yorkers, roughly 14 percent of state residents were found to have been exposed to the virus — and 1 in 5 in New York City. Some proponents of such tests say they could pave the way for “immunity passports,” documents identifying people who have previously been infected and may now be immune to the virus. They might allow people to rejoin the workforce, or eat in a restaurant.